Frequency selective surfaces are useful in a number of applications. Such applications include radomes, canopies, and other aircraft structures and the receiving surfaces of satellite dishes. A surface may be made frequency selective by forming a pattern on the surface, for example, by applying a patterned metal layer to the surface. The accuracy of the frequency selectivity of the surface depends on the precision of the pattern formed on the surface. Any curvature in the surface complicates the pattern and makes the achievement of precise frequency selectivity extremely difficult. This is especially true in the case of complexly curved surfaces. Currently, there is no known method for patterning curved surfaces to achieve precise frequency selectivity in a cost effective manner.
One method that has been tried is splicing flat sheets of etched copper onto a complexly curved surface. This method has produced inadequate alignment of the elements of the pattern and resulting unacceptable inaccuracies in the overall pattern.
The present invention is directed toward a mask useful in a conformal photolithographic system that solves the problems discussed above and makes possible the efficient manufacture of parts having curved frequency selective surfaces. The system of the invention has various aspects, including a method of manufacturing parts, parts made by such method, a mask for use in manufacturing parts, and a method of manufacturing the mask. In the system, use of the mask in the manufacture of the parts is a key feature. It provides consistency in the quality and surface pattern-related characteristics of the parts, and allows the overall system to be applied in a highly cost effective manner.
The invention provides a method of making a part having a predetermined shape that includes a curved surface, and of forming a pattern on the surface, which the method comprises providing a mask having a shape complementary to the curved surface, transparent portions transparent to electromagnetic radiation with a predetermined range of frequencies, and opaque portions opaque to such radiation. The method further comprises forming a part body having said predetermined shape. A layer of metal is applied to the curved surface of the part body, and a layer of photosensitive material is applied over the layer of metal. After these layers have been applied to the part body, the mask and the part body are mated, using a vacuum bag to achieve intimate contact between the mask and the layer of photosensitive material. With the mask and the part body mated, the layer of photosensitive material is exposed through the mask to radiation having a range of frequencies within the predetermined range, to define selected areas of the layer of photosensitive material. Then, the layer of photosensitive material is chemically developed to remove it from the selected areas. The layer of metal is also removed from the selected areas.
The step of exposing the layer of photosensitive material may be carried out in various ways. Preferably, it comprises exposing the layer of photosensitive material to essentially parallel radiation. Such exposure may be accomplished, for example, by exposing the layer to a light source relatively far away from the layer through an aperture close to the layer, or by exposing the layer to radiation from a laser. The use of a distant non-laser light source is generally preferred because it allows the exposing step to be completed relatively quickly and inexpensively. As used herein, the term "essentially parallel radiation" means radiation that produces a sharp boundary between illuminated area and shadow when it is projected through an aperture. The degree of sharpness required in a particular situation depends on the required tolerances for that situation.
In the method of making the part, the mask may take various forms. In the preferred embodiments, the mask includes a substrate transparent to radiation having the predetermined range of frequencies suitable for exposing the photosensitive material. The transparent portions and the opaque portions are defined by a discontinuous layer, on the substrate, of material opaque to radiation having the predetermined range of frequencies. In a first preferred embodiment, the discontinuous layer comprises paint. In a second preferred embodiment, the discontinuous layer is metallic. Whatever the nature of the material forming the discontinuous layer, the configuration of the layer may vary, and as used herein, the term "discontinuous layer" includes a layer comprising a plurality of separate unconnected elements, a layer comprising a plurality of elements at least some of which touch each other, and a layer having a continuous background interrupted by holes in the layer.
Another feature of the invention is to provide a part made by the above-described method.
Still another object of the invention is to provide a method of making a mask for use in forming a pattern on a plurality of curved surfaces of a predetermined shape. According to an aspect of the invention, the method comprises forming a substrate from a material transparent to electromagnetic radiation within a predetermined range of frequencies. The substrate has a shape complementary to the predetermined shape. A layer of material opaque to radiation within the predetermined range of frequencies is applied to the substrate. Essentially parallel electromagnetic radiation is used to define temporary areas and permanent areas of the layer, and the temporary areas of the layer are removed while leaving the permanent areas. The material forming the layer may, for example, comprise paint or be metallic. The temporary areas of either of these two types of opaque material layers may be simultaneously defined and removed by laser ablation.
In the case of a metallic layer, an alternative procedure is to apply a second layer of photosensitive material over the metallic layer before defining the temporary and permanent areas. The step of defining the temporary and permanent areas comprises subjecting portions of the second layer to essentially parallel electromagnetic radiation. These portions of the second layer may be laser ablated. Alternatively, these portions may be laser cured. As used herein, the term "laser cured" in reference to a material means that the material is subjected to a laser to change the material's molecular structure to thereby change the material's solubility in a chemical.
The present invention provides a mask for use in forming a pattern on a plurality of curved surfaces. The mask is preferably made in accordance with the method described above. According to an apparatus aspect of the invention, the mask is designed for forming a pattern on a plurality of complexly curved surfaces. The mask comprises a substrate which is transparent to electromagnetic radiation within a predetermined range of frequencies and which has a shape complementary to the complexly curved surfaces. The mask further comprises a discontinuous layer, on the substrate, of material opaque to such radiation. The layer defines transparent portions of the mask transparent to such radiation and opaque portions of the mask opaque to such radiation, to define the pattern. As noted above, the discontinuous layer may be formed from various materials, such as paint or metal.
The mask of the invention is suitable for forming a wide variety of patterns. In one embodiment of the mask, the pattern comprises a multiplicity of substantially identical elements that are repeated throughout the pattern. The transparent portions and the opaque portions define the elements with some of the elements rotated to compensate for curvature. The elements per se may be either transparent or opaque. The term "substantially identical" is intended to be understood as including elements with essentially the same shape and the same or different sizes and the same or different orientations. The sizes of the elements may vary by a factor of two or more. The elements may be touching or separated from each other. In another embodiment of the mask, the transparent and opaque portions define a circuit.
The system provides a cost effective solution for manufacturing a number of essentially identical frequency selective surfaces that meet precise performance requirements. In the system of the invention, the patterning of even complexly curved surfaces to give the surfaces precise frequency selective characteristics can be accomplished at a reasonable cost. The use of a mask in the system of the invention allows the precision of the patterning to be built into the mask so that the manufacture of each patterned part requires relatively little labor and time. The method of the invention has sufficient versatility and flexibility so that virtually any required degree of precision can be accomplished. Preferred features of the invention that serve to help maximize precision are the use of a laser in the manufacture of the mask and/or the parts. By making possible the manufacture of complexly curved surfaces with highly precisioned frequency selective patterns thereon, the invention in turn allows much greater freedom in the design of the structures into which the surfaces are to be incorporated. For example, surfaces which in the past have had a relatively flattened configuration in order to accomplish the desired frequency selectivity can, by use of the invention, be made with more highly curved configurations to meet other performance requirements while preserving the frequency selectivity.
These and other advantages and features will become apparent from the detailed description of the best modes for carrying out the invention that follows.